Interpretive Summary: The flowers of Canada thistles, Cirsium arvense (L.)attract a wide range of insects including pollinators and herbivorous species. This attraction is primarily mediated by floral odor, which offers potential for developing generic insect attractants based on odor. In this study we have analyzed the chemical composition of the volatiles produced by Canada thistle flowers. Nineteen floral compounds were identified in the headspace
including: phenylacetaldehyde (55%), methyl salicylate (14%), dimethyl salicylate (8%), pyranoid linalool oxide (4.5%), and benzaldehyde (3.5%). Other minor compounds include benzyl alcohol, methylbenzoate, linalool, phenylethyl alcohol, furanoid linalool oxide, p-anisaldehyde, 2,6-dimethyl-1,3,5,7-octatetraene, benzylacetate, benzyl tiglate, alpha-farnesene, benzyl benzoate, isopropyl myristate, and 2-phenylethyl ester benzoic acid. The relative attractiveness of various doses of the main floral volatile compound phenylacetaldehyde (i.e. 10, 100, 200, and 400 mg) was tested for insect attraction. Both the total catch and the biodiversity of insect species trapped increased as the loading of phenylacetaldehyde increased. Various blends of volatiles produced by flowers of many plant species including Canada thistle were formulated and tested in the field. An 11-component blend was the most attractive blend of several tested. These findings indicate that chemical components of flower odors of Canada thistle can serve as a generic insect attractant for monitoring of invasive pest species.

Technical Abstract:
The flowers of Canada thistles, Cirsium arvense (L.)attract a wide range of insects including pollinators and herbivorous species. This attraction is primarily mediated by floral odor, which offers potential for developing generic insect attractants based on odor. In this study we have analyzed the chemical composition of the volatiles produced by Canada thistle flowers. Nineteen floral compounds were identified in the headspace
including: phenylacetaldehyde (55%), methyl salicylate (14%), dimethyl salicylate (8%), pyranoid linalool oxide (4.5%), and benzaldehyde (3.5%). Other minor compounds include benzyl alcohol, methylbenzoate, linalool, phenylethyl alcohol, furanoid linalool oxide, p-anisaldehyde, 2,6-dimethyl-1,3,5,7-octatetraene, benzylacetate, benzyl tiglate, alpha-farnesene, benzyl benzoate, isopropyl myristate, and 2-phenylethyl ester benzoic acid. The relative attractiveness of various doses of the main floral volatile compound phenylacetaldehyde (i.e. 10, 100, 200, and 400 mg) was tested for insect attraction. Both the total catch and the biodiversity of insect species trapped increased as the loading of phenylacetaldehyde increased. Various blends of volatiles produced by flowers of many plant species including Canada thistle were formulated and tested in the field. An 11-component blend was the most attractive blend of several tested. These findings indicate that chemical components of flower odors of Canada thistle can serve as a generic insect attractant for monitoring of invasive pest species.